Years ago, we thought this battle was over, and we thought Microsoft won. But Apple changed the PC game with the iPad, and now Windows is roaring back. The Mac/PC battle has entered a new phase.

There are three main kinds of personal computers. Traditional desktops and laptops are dominated by Windows. Smartphones are a jumble of operating systems. But tablets are growing fast, and they're eating up the low-end of the laptop market. Led by the iPad, the tablet is becoming a mainstream, entry-level computer for many people.

Now Microsoft wants a bite of that market: so, Windows on ARM, or WOA. There have been Windows tablets for many years, but because of the limitations of Intel's hardware and Microsoft's software, they haven't been cheap enough, light enough, or easy enough to use to grab the iPad consumer.

WOA promises to change that by porting Windows to iPad-like hardware. The ARM chip architecture is what all of those iPads and Android tablets use, and there's no reason Windows won't be able to run on something with the specs of an iPad or an Asus eee Pad, going for under $500.

WOA is essentially a separate experience from Windows 8, though, which means that from users' perspective, Microsoft will have three overlapping operating systems: Windows 8, WOA, and Windows Phone.

WOA tablets won't run the hundreds of thousands of apps that currently run on Windows PCs. Because those apps are compiled for Intel's chips, they'll have to be rewritten for the new tablets.

For a while, that means we'll have two incompatible sets of computers running "Windows 8": Intel-based computers that can run all the old Windows 7 apps, and WOA tablets that may be lighter and cheaper, but can only run new apps, and only those downloaded through the Windows Store.

Notice that last bit. WOA tablets will only be able to download apps from Microsoft's official store, just like the iPad can only download apps from iTunes, and unlike current PCs.

This offers a big opportunity to Intel, if it can get its act together to provide chips that compete with ARM on size, price, and power consumption. Intel-powered, low-cost tablets wouldn't suffer the restrictions imposed on WOA; they'd be "normal" Windows devices, able to run most Windows apps. On the other hand, remember, most of those apps aren't designed for touch screens anyway.

Will Microsoft or Google Best Battle Apple?
In the world of tablets, Apple is the new "PC" - the universal standard by which all others are judged, the one for which apps are written first, and the one which you might be seen as a weirdo not to choose.

But the mainstream tablet world is new. If this was the PC world, we'd be in 1984 or 1985 right now. And Google and Microsoft are now angling to be the "Mac" to Apple's tablet "PC."

As they rush to compete, the real question will be around apps. Windows on ARM is starting with Microsoft Office, which is a big deal, but after that it's basically at zero. Microsoft must convince app developers that there's enough of a potential market to write their apps for Windows on ARM.

Google's tablet struggles have shown what happens when you can't get enough developers on board. Google has done a pretty poor job of getting apps written specifically for Android tablets (rather than designed primarily for smaller phone screens), which has contributed to slow sales (at least compared to the iPad) of even powerful, high-quality tablets like the Asus eee Pad Transformer Prime.

WOA promises great peripheral support and plenty of products coming from mainstream PC manufacturers. But by the time WOA tablets arrive later this year, they'll face up against a very established iPad ecosystem and an Android tablet ecosystem that may finally be finding its footing.

I'd never bet against Microsoft, which is very rich, very patient, and has a lot of PC manufacturers behind it who see their future in tablets. But it'll be a long road to build the library of apps which would make WOA tablets an iPad alternative for the average consumer.

PCMag.com's lead mobile analyst, Sascha Segan, has reviewed hundreds of smartphones, tablets and other gadgets in more than 9 years with PCMag. He's the head of our Fastest Mobile Networks project, one of the hosts of the daily PCMag Live Web show and speaks frequently in mass media on cell-phone-related issues. His commentary has appeared on ABC, the BBC, the CBC, CNBC, CNN, Fox News, and in newspapers from San Antonio, Texas to Edmonton, Alberta.
Segan is also a multiple award-winning travel writer, having contributed...
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